I've used the video2mp3 site in the past and it works well.
These sites and add-ons are probably only legal because of some loophole in the law (keep in mind I'm no lawyer and am happy to be corrected on this) that allows them to do this because it's assumed that any video/soundtrack etc uploaded to the sites they offer these services too are for non-copywritten material (i.e. personal, which is essentially why youtube started). I'd imagine it's youtube's responsibility to make sure that copywritten material is deleted (and they have on a few occasions disabled the soundtrack or deleted videos for that very reason).
But I reckon you'd be safe converting them for personal use, assuming you don't find it objectionable. Any company looking for financial recompense would be starting legal battles with the person who uploaded it first, then the service that made it public, and then the service that allowed you to convert it. Legally, I doubt you have much to worry about, morally... well, that's your call.